As an optical imaging technique, photoacoustic tomography (PAT) has been proposed lately.
If a living body (object) is irradiated with light, such as pulsed laser light, an acoustic wave is generated when the light is absorbed by biological tissue inside the object. This phenomena is called a “photoacoustic effect”, and an acoustic wave generated by the photoacoustic effect is called a “photoacoustic wave”. The tissues that constitute an object have different light energy absorption rates respectively, hence the sound pressure of the photoacoustic wave to be generated from each tissue is also different. In PAT, the generated photoacoustic wave is detected by a probe, and the detected signal is mathematically analyzed, whereby an optical characteristic inside the object, particularly the distribution of the light energy absorption density, can be imaged.
A major technique to calculate the initial sound pressure of the acoustic wave generated inside the object is the back projection method. The initial sound pressure P0 of the acoustic wave generated from a light absorber inside the object is given by Expression (1).P0=Γ·μa·Φ  Expression(1)
Here Γ is a Gruneisen coefficient, and is determined by dividing the product of the volume expansion coefficient β and the square of the sound velocity c by a specific heat at constant pressure Cp. It is known that Γ is approximately constant once the object is determined. μa is a light absorption coefficient of an absorber, and Φ is a light fluence [J/m2 or J/m3] in a local region of the object.
Patent Literature 1 discloses a technique to measure by an acoustic wave detector the temporal change of the sound pressure P of an acoustic wave propagated through an object, and calculate the initial sound pressure distribution based on the measurement result. By dividing the calculated initial sound pressure distribution by the Gruneisen coefficient Γ, the product of μa and Φ, that is, the absorption density of the light energy, can be acquired.
The Gruneisen coefficient is approximately constant for each object, hence the light fluence distribution inside the object must be determined in order to acquire the distribution of the light absorption coefficient μa from the distribution of the initial sound pressure P0.